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But these compulsions aren't inscribed in our genes or hard-wired into the brain at birth. Scientists are discovering that habits are simply an extreme form of learning, a behavior that's so familiar we no longer need to think about it.

The malleability of habits isn't news to Madison Avenue: Effective commercials show how people can be quickly trained to do something new and then keep on doing it. The secret, it turns out, is the quick combination of a memorable cue and a rewarding experience.

Consider Febreze, a product designed by Procter & Gamble in the 1990s to remove bad odors. As Charles Duhigg recounts in his fascinating new book, "The Power of Habit," Febreze underperformed in early tests and was in danger of being canceled. Consumers couldn't fathom what the product was for.

Febreze didn't become a superstar until the P&G marketing team created an ad campaign based on habit formation. The television spots showed homemakers performing a chore—making a bed, mopping the kitchen—and then spritzing a little scented Febreze into the air. The spritz was always followed by a big smile.

What's most interesting is that instead of focusing on removing bad smells, the ads set up Febreze—to which perfume had been added—as the reward for a bout of cleaning, satisfying the desire to make things smell nice, not just look good. The ads taught consumers a new habit, training them to associate the rewarding positive cue—a spotless space—with the use of Febreze. Before long, the product was a best seller.

Now we can see how these habits take hold in our brains. In a new paper, the neuroscientist Joe Z. Tsien and colleagues at Georgia Health Sciences University describe a mutant strain of mice that were incapable of developing new habits. While ordinary mice quickly developed the habit of pressing a lever to get a food pellet (leading to overeating), the mutant mice stopped pressing the lever as soon as they felt full.

These mice were missing a protein known as an NMDA receptor on their dopamine neurons. Normally, these receptors help to generate a big electrical response when an animal is repeatedly exposed to a rewarding cue, such as a food pellet. According to Mr. Tsien's data, this specific response is what transforms ordinary learning into an automatic behavior. It doesn't matter if we're learning to overeat or to spray Febreze. That big signal from the NMDA receptor makes the difference.